1. Field of the Invention
The present invention relates to a plasma processing apparatus.
2. Description of the Related Art
Plasma processing apparatuses turn gas supplied to a processing chamber into plasma using high frequency power to perform a plasma process on a processing object. One known gas supplying method involves showering the gas into the processing chamber from an upper electrode. Further, in the case of implementing such a gas supplying method, a technique is known that involves arranging multiple gas chambers (gas supplying zones) at an edge side and a center side of the upper electrode and supplying a gas mixture of different types of gases to the processing chamber via the gas chambers (see e.g., Japanese Laid-Open Patent Publication No. 2006-165399).
In Japanese Laid-Open Patent Publication No. 2006-165399, different types of gases are mixed at a mixing pipe and then branched out to multiple branch pipes so that the mixed gas may be supplied to the gas chambers. In this case, by adding an adjustment gas for accelerating the etching rate to one of the branch pipes, for example, a certain gas chamber may supply a processing gas having the adjustment gas added thereto while another gas chamber may supply processing gas without the adjustment gas.
By adjusting the amount of adjustment gas contained in gas that is introduced into the processing chamber from the gas chambers in this manner, variations in the etching rate may be reduced and etching uniformity of a substrate surface may be improved, for example.
However, when an adjustment gas with a relatively large molecular weight is added to a processing gas including a main gas with a relatively small molecular weight, it is difficult to mix the main gas and the adjustment gas together due to their differences in molecular weight. In such case, the gases may be introduced into the processing chamber from a gas shower head before they are adequately mixed together, and so, variations may occur in the mix ratio of the gas introduced into the processing chamber. As a result, in performing a plasma process on a processing object, processing uniformity may be compromised due to variations in the gas concentration of the gas supplied to the processing chamber.
In this respect, the branch pipe into which the adjustment gas is added may be increased in length so that adequate reaction time may be secured for the gases to be completely mixed before they are introduced into the processing chamber. However, a flow rate adjusting apparatus (flow splitter) is arranged to control the flow rate of gas that is branched out to a branch pipe by monitoring the pressure within the branch pipe after the gas is branched out. Accordingly, in order to enable the flow rate adjusting apparatus to promptly monitor pressure variations within the branch pipe after the gas is branched out and reflect the monitoring result in gas flow rate control operations, the length of the branch pipe into which the adjustment gas is added is preferably arranged to be as short as possible. Thus, considering the need to ensure accurate flow rate control by the flow rate adjusting apparatus, the length of the branch pipe cannot be increased to a length that would secure enough reaction time for the gases to be completely mixed. Further, simply increasing the length of the branch pipe linearly may not be compatible with aims to achieve a more compact design and miniaturization of the plasma processing apparatus.